um: Print minimum physical memory requirement

Print a more sensible message about the minimum physical memory
requirement.
Signed-off-by: NThomas Meyer <thomas@m3y3r.de>
Signed-off-by: NRichard Weinberger <richard@nod.at>

arch/um/kernel/physmem.c

@@ -57,22 +57,51 @@ void map_memory(unsigned long virt, unsigned long phys, unsigned long len,
 
 extern int __syscall_stub_start;
 
+/**
+ * setup_physmem() - Setup physical memory for UML
+ * @start:	Start address of the physical kernel memory,
+ *		i.e start address of the executable image.
+ * @reserve_end:	end address of the physical kernel memory.
+ * @len:	Length of total physical memory that should be mapped/made
+ *		available, in bytes.
+ * @highmem:	Number of highmem bytes that should be mapped/made available.
+ *
+ * Creates an unlinked temporary file of size (len + highmem) and memory maps
+ * it on the last executable image address (uml_reserved).
+ *
+ * The offset is needed as the length of the total physical memory
+ * (len + highmem) includes the size of the memory used be the executable image,
+ * but the mapped-to address is the last address of the executable image
+ * (uml_reserved == end address of executable image).
+ *
+ * The memory mapped memory of the temporary file is used as backing memory
+ * of all user space processes/kernel tasks.
+ */
 void __init setup_physmem(unsigned long start, unsigned long reserve_end,
 			  unsigned long len, unsigned long long highmem)
 {
 	unsigned long reserve = reserve_end - start;
-	int pfn = PFN_UP(__pa(reserve_end));
-	int delta = (len - reserve) >> PAGE_SHIFT;
-	int err, offset, bootmap_size;
+	unsigned long pfn = PFN_UP(__pa(reserve_end));
+	unsigned long delta = (len - reserve) >> PAGE_SHIFT;
+	unsigned long offset, bootmap_size;
+	long map_size;
+	int err;
+
+	offset = uml_reserved - uml_physmem;
+	map_size = len - offset;
+	if(map_size <= 0) {
+		printf("Too few physical memory! Needed=%d, given=%d\n",
+		       offset, len);
+		exit(1);
+	}
 
 	physmem_fd = create_mem_file(len + highmem);
 
-	offset = uml_reserved - uml_physmem;
 	err = os_map_memory((void *) uml_reserved, physmem_fd, offset,
-			    len - offset, 1, 1, 1);
+			    map_size, 1, 1, 1);
 	if (err < 0) {
 		printf("setup_physmem - mapping %ld bytes of memory at 0x%p "
-		       "failed - errno = %d\n", len - offset,
+		       "failed - errno = %d\n", map_size,
 		       (void *) uml_reserved, err);
 		exit(1);
 	}